Re: paging question

Cesar Suga
2002-02-20T19:56:40Z

On Wed, 20 Feb 2002, Jason Yan wrote:

> I have a question about the code to enable to initialize page
> tables in linux/arch/i386/head.S

> I search the internet again and again but fail to find any answer
> so far, maybe you gurus can help me out, here it goes:

> 48 cld
> 49 movl $(__KERNEL_DS),%eax
> 50 movl %eax,%ds
> 51 movl %eax,%es
> 52 movl %eax,%fs
> 53 movl %eax,%gs
> 81 /*
> 82 * Initialize page tables
> 83 */
> 84 movl $pg0-__PAGE_OFFSET,%edi /* initialize page tables */
> 85 movl $007,%eax /* "007" doesn't mean with right to kill, but
> 86 PRESENT+RW+USER */
> 87 2: stosl
> 88 add $0x1000,%eax
> 89 cmp $empty_zero_page-__PAGE_OFFSET,%edi
> 90 jne 2b
>
> I remove the SMP code. According the setup.S, gdt_table is setup as
> gdt_table:
> #.quad 0x0000000000000000; // null
> #.quad 0x0000000000000000; // not used
> #.quad 0x00cf9a000000ffff; // 0x10 kernel 4GB code at 0x00000000
> #.quad 0x00cf92000000ffff; // 0x18 kernel 4GB data at 0x00000000
>
> 1) So, what's in %eax after line 49 ? 0x0 ?

As in asm/segment.h (included on arch/i386/kernel/head.S): (i386)

#ifndef _ASM_SEGMENT_H
#define _ASM_SEGMENT_H

#define __KERNEL_CS 0x10
#define __KERNEL_DS 0x18

uaccess.h specifies another value, but that's KERNEL_DS for user
space memory.

> 2) Isn't __PAGE_OFFSET 0xC0000000 ? what's the result of $pg0-__PAGE_OFFSET ?

Yes, __PAGE_OFFSET is 0xC0000000. I am not sure, but I think it
initializes the page tables to 8MB here, and the final page tables depend
on the memory size (better seeing pgtable.h)...

Regards,
Cesar Suga <sartre@linuxbr.com>